In order to control the motion states of a synchronous machine in a stable manner it is required to measure the rotor position. This can be achieved by mounting mechanical devices such as encoders or resolvers on the shaft of the machine or by placing hall-sensors or search coils within the machine. Due to important temperature variations within the machine, as well as mechanical vibrations, these sensors can fail, consequently reducing the reliability of the controlled machine. For this reason, as well as to reduce cost and extra cabling, the motion states are estimated instead of measured. This is frequently done by an estimation of the speed dependent induced voltage. However, the signal-to-noise ratio of this voltage is too poor at low speeds to estimate the motion states. Moreover, at standstill, the rotor position can't be estimated from the induced voltage.
At low speeds and at standstill, it is known to inject high-frequency signals in addition to the waveforms required for normal operation of the machine. As the machine has a reluctance variation along the air gap due to the construction of the rotor or due to saturation of the magnetic path, the high-frequency response will depend on the rotor position. By processing this response, an accurate estimation of the rotor position can be made. A parameterless sensorless strategy for the low speed region is disclosed in U.S. Pat. No. 5,565,752. As high-frequency sinusoidal test signals are applied to the machine, band pass filters and a quadrature demodulation or heterodyne detection process are used to obtain an estimation of the rotor position. These filters can generate an important phase delay in the control loop, reducing the dynamic behavior of the drive.
Another parameter-less sensor-less method which reduces the number of filters is disclosed in U.S. Pat. No. 5,796,235, where high-frequency voltage test pulses have been supplied to the machine. Also a further enhancement thereof is known to minimize the current disturbances generated by the test pulses. Additional test pulses with a minimum time span have been computed in order to reduce the disturbance in the steady-state current. The reason for the current disturbances is the variation of the steady-state voltage vector during the test-sequence. Despite the advantage of reducing current disturbances, a machine model has to be identified in order to compute the additional test sequence which lowers the current disturbance.
A parameter-less sensor-less method which injects pulses into the machine terminals without altering the steady-state voltage is disclosed in US2001/002784A1. Nevertheless, in this method the estimated rotor position isn't obtained in one sampling period but converges in several periods to the real one through iteration.
Again another parameterless sensorless method is disclosed in U.S. Pat. No. 6,051,946 which relies on varying, during a test sequence, the voltage by applying high-frequency pulses and maintaining the steady-state. Moreover the estimation error due to saturation is reduced by using a look-up table. Nevertheless, in this method identification procedures are required to obtain the look-up table.
A method for determining rotor position involving provision of test space vectors is also described in DE10226974, the method comprising deriving an estimated rotor position and an error angle for correcting the estimated rotor position.
There is still a need for good methods and systems for controlling salient-pole machines.